The present invention is concerned generally with a system and method for monitoring processes and systems producing any type of spectra as well as ones having signals with periodic components. More particularly, the invention is concerned with a system and method for combining a sensitive probabilistic methodology operating in the time domain with an analysis of signals in a parametric space to provide a three-dimensional surface plot enabling sensitive monitoring for onset of subtle disturbances in signal spectra. Such a process and system permits analysis of deviations from normal operation or identification of a particular classification or trend of signal behavior.
Conventional parameter-surveillance schemes are sensitive only to gross changes in the mean value of a process, or to large steps or spikes that exceed some threshold limit check. These conventional methods suffer from either large numbers of false alarms (if thresholds are set too close to normal operating levels) or a large number of missed (or delayed) alarms (if the thresholds are set too expansively). Moreover, most conventional methods cannot perceive the onset of a process disturbance or sensor deviation which gives rise to a signal below the threshold level for an alarm condition.
In other prior art surveillance systems, the periodic components of signals from equipment or processes are monitored using Fourier analysis (most commonly as implemented in the Fast Fourier Transform, or xe2x80x9cFFTxe2x80x9d). The FFT produces a power spectral density (xe2x80x9cPSDxe2x80x9d) function for a signal that exhibits peaks at frequencies associated with periodic phenomena. By watching the amplitude of these peaks, or by watching for changes in the frequencies associated with these peaks, it is possible in many cases to infer diagnostic information related to the health of the system being monitored. Some drawbacks of these conventional systems that are based on examination of PSDs include: (1) most conventional systems rely on a human to examine the PSD spectra to look for changes in the abscissa or ordinate of peaks of interest; and (2) if one tries to automate the surveillance of PSD information by assigning thresholds for the amplitudes of various peaks of interest, then one encounters the conventional problem of false and missed alarms. That is, if one sets the thresholds at low values, it is possible to trip alarms from minor idiosyncrasies in the signals that may have no safety or operational significance whatsoever. If one tries to avoid this problem by setting the thresholds further apart, then the degradation in the equipment or process can be much worse.
In another conventional monitoring method, the Sequential Probability Ratio Test (xe2x80x9cSPRTxe2x80x9d) has found wide application as a signal validation tool in the nuclear reactor industry. Two features of the SPRT technique make it attractive for parameter surveillance and fault detection: (1) early annunciation of the onset of a disturbance in noisy process variable, and (2) the SPRT technique has user-specifiable false-alarm and missed-alarm probabilities. One important drawback of the SPRT technique that has limited its adaptation to a broader range of applications is the fact that its mathematical formalism is founded upon an assumption that the signals it is monitoring are purely Gaussian, independent (white noise) random variables.
It is therefore an object of the invention to provide an improved apparatus and method for monitoring the condition of a system characterized by periodic signals and/or a source of data for the signals.
It is also an object of the invention to provide a novel apparatus and method for continuous evaluation and interactive modification of a system.
It is a further object of the invention to provide an improved system and method for identifying a deviation from normalcy of a system generating substantially periodic signals.
It is an additional object of the invention to provide a novel system and method for characterizing the class of signal behavior.
It is yet another object of the invention to provide an improved system and method which combines a probabilistic analysis of periodic signals in time space with frequency spectra to produce a three-dimensional characteristic plot enabling sensitive analysis of the signals.
It is a further object of the invention to provide a novel apparatus and method which analyzes periodic signals using a sequential probability ratio test and Fourier transformed spectra (or other types of spectra) to produce a three-dimensional surface indicative of the state of the underlying system producing the signals.
It is yet an additional object of the invention to provide an improved apparatus and method which combines time domain and frequency domain analysis of periodic signals to produce a three-dimensional classification surface characteristic of the operating state of a system.
It is also an additional object of the invention to provide a novel apparatus and method for combining probabilistic analysis of periodic signals with frequency domain analysis of signals to identify deviations from a desired behavior of a system or of the sensors sensing the signals or other source providing the signals.
It is in addition another object of the invention to provide an improved apparatus and method for processing periodic signal databases using a combined time and frequency domain analysis.
It is also a further object of the invention to provide a novel apparatus and method for removing statistically certain noise signals from a periodic signal after combining time domain information with a frequency domain transformation of the information to enable sensitive signal analysis.
It is yet another object of the invention to provide an improved apparatus and method to operate on a substantially periodic signal (or other different types of spectra) to remove unwanted serial correlation noise after combining reduced time data with frequency domain data (or spectral data with frequency data) to enhance sensitivity to deviations from desired behavior or to classify or establish a trend of the underlying system.
It is still another object of the invention to provide a novel apparatus and method for applying a pattern recognition methodology to a power spectral density (xe2x80x9cPSDxe2x80x9d) function incrementally taken over a plurality of time windows to yield a three-dimensional surface function characteristic of the operating state of a system or the source of signals from the system.
Other objects, features and advantages of the present invention will be readily apparent form the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings described below.